Method of protecting a heat-resistant element from hot media



March 7,1967 .J.GIGER 3,307,616

METHOD OF PROTECTING A HEAT-RESISTANT ELEMENT FROM HOT MEDIA Filed Nov. 27, 1964 5 Sheets- Sheet 1 March 7,1967 JQGIGER 3,307,616

METHOD OF PROTECTING A HEAT-RESISTANT ELEMENT FROM HOT MEDIA Filed Nov. 27. 1964 I 5 Sheetsheet 2 J. GIGER 3,307,616 ME'II'I IOD OF PROTECTING A HEAT-RESISTANT ELEMENT FROM HOT MEDIA- March 7, 1967 5 Sheets-Sheet 5 Filed Nov. 27, 1964 FIGS United States Patent 3,307,616 METHOD OF PROTECTING A HEAT-RESISTANT ELEMENT FROM HOT MEDIA Johannes Giger, Baden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Filed Nov. 27, 1964, Ser. No. 414,279 Claims priority, application Switzerland, Nov. 28, 1963, 14,581/ 63 9 Claims. (Cl. 165-1) This invention relates to a method of protecting those parts of the surface of the heat-resistant element which are swept by hot media.

The walls of combustion chambers the inside surfaces of which are directly exposed to flame naturally have relatively short lives unless they are very well cooled. Various cooling systems have been disclosed to prolong the life of combustion chambers or elements having walls swept by hot media. The present invention is intended for this purpose. The heat-resistant element according to the invention is characterized in that at least part of the element is gas-permeable so that a coolant fluid can be moved through said part for the purpose of cooling the element.

The method of protecting a heat-resistant element according to the invention is characterized in that a coolant fluid is blown through the gas-permeable part thereof transversely into the flow of hot medium.

The invention will now be explained by way of example with reference to the drawings, wherein:

FIG. 1 is a perspective view of a cooling system including a part of a combustion chamber wall;

FIG. 2 is a section of the part of the combustion chamber wall on the line IIII in FIG. 1;

FIG. 3 is a perspective View of part of a cylindrical combustion chamber Wall;

FIG. 4 is a perspective view of a porous heat-resistant element with a connection for cooling medium, and

FIG. 5 is a section through a combustion chamber structural element with a cooling system.

The combustion chamber wall 1 shown in FIGS. 1 and 2 comprises an inner wall element 3 and an outer wall element 5. The inner wall element 3 consists of sintered-together fragments 7 while the outer wall element 5 is made up of shaped bricks 9 as will be apparent from FIG. 1. Injection tubes 11 are disposed slidably in apertures 37 in the shaped bricks 9. These tubes 11 lead to a header 13 fed by an air supply pipe 31. The pipe 31 branches into a T-piece which leads to valves 27 and 29 from each of which a pressure line 25 runs, said pressure lines 25 extending respectively directly to the header pipe 13 and indirectly to said pipe 13 via a mixing tank 15 and a feed line 23.

A perforate distributor element 17 above which is a layer of loose ceramic powder 21 is provided in the base 19 of the mixing tank 15. A nozzle 33 is also shown on the inside of the combustion chamber with fuel emerging therefrom to produce a flame 35 on ignition. As a result of the flame 35 the jets of cooling medium emerging from the injection tubes 11 are deflected in the direction of the arrows 39.

During operation of the combustion chamber its inner part in particular, i.e., the inner wall element 3, undergoes increasing wear with increasing temperatures in the combustion chamber. To reduce the temperature influence, cooling air is forced directly into the injection tubes 11 during operation via the compressed air supply pipe 31, the valve 29, the compressed air pipe 25 and the header 13. The cooling air flows through the combustion chamber wall 1 and acts as a cooling screen for the inner surface of the wall. Because of its low temperature it protects the combustion chamber wall from the 3,307,616 Patented Mar. 7, 1967 influence of the flame 35. The axes of the streams of cooling air flowing through tubes 11 are at least approximately perpendicular to the combustion chamber wall to be cooled.

It has been found that such cooling is in many cases inadequate and the inner wall element 3 undergoes considerable wear and the protective effect gradually diminishes. To compensate for this, the valve 29 is closed and compressed air is fed from the air supply pipe 31, through the valve 27 and the compressed air pipe 25, to the perforate distributor element 17, where the air is divided up into fine streams which flow into the ceramic powder 21 and fluidize the same depending upon their speed of exit. Some of the fluidized ceramic powder is entrained by the passing air and flows through the feed pipe 23 and the header 13 to the injection tubes 11 and then to the interior of the combustion chamber together with the cooling air. Because of their inertia, the particles shoot much farther into the interior of the combustion chamber than the air transporting the particles, so that they reach the region of the flame 35 where at least their outer surfaces are so heated that when they subsequently meet the inner wall elements 3 they lodge thereon and thus form a coating of material on the wall.

The return of the ceramic particles, whose surfaces are softened by heat, to the surface of the wall element 3 is governed by the turbulent mixing movement and the spreading mixing zone of the flame jet and the air streams which emerge into the combustion chamber through the tubes 11. The wear of the inner wall element 3 can thus be compensated and any such wear can be replaced by appropriate regulation of the amount of entrained ceramic powder. The inner wall element 3 may, for example, be formed of porous shaped bodies of frag ments 7 of A1 0 and may be made in the following way:

Ceramic balls of A1 0 with a diameter of 6 mm. for example are shaken into a mold of ceramic material (A1 0 or graphite and joggled to form a compact mass. The mold with the layer of balls is then sintered at about 1650 C. for 3 hours, for example, to form a block. Where a ceramic mold is used the sintering can be effected in air while when the sintering is carried out in a graphite mold a protective gas, for example nitrogen is used. The walls and base of the ceramic mold must be removed after firing. It is also possible to make porous wall elements by sintering together chromium or other metal cylinders, bal-ls or cubes at 1700 C. in a vacuum or in a protective gas atmosphere in a ceramic or graphite mold. The sintering of chromium balls or other chromium elements can be assisted by previously vapor-coating the surfaces of these elements with another metal, for example nickel. Alternatively, to provide inner wall element 3, open tubes of MgO, for example of a size of 12 x 9 mm. and a length of 100 mm., can be superimposed and, after pre-coating the contacting places with high-temperature cement, fired to form porous wall elements by sintering at l'0 C. in air.

Such shaped elements can be assembled in variou ways. 'For example, ceramic tiles are advantageously joined with high-temperature cement. Shaped metal or cement parts can be combined by arranging for the same to interlock by means of stepped portions or grooves on the prefabricated component assembly principle.

The apertures are so distributed in the combustion chamber wall that the inner wall element 3 is covered and protected by the cooling air streams throughout. It has been found advantageous to make the air flow between about 5 and 10 litres per second and per square dm. of inner wall area. The porous inner wall element 3 as shown in FIGS. 1 and 2 may also be made from nOnmetallic materials (oxides), for example aluminum oxide,

beryllium oxide, magnesium oxide, zirconium oxide, thorium oxide or mixtures of different high-melting ceramic oxides. Various carbides, nitrides, borides, silicides and sulphides are also suitable for the production of such inner wall parts, for example silicon carbide, boron carbide, boron nitrides, molybdenum disilicide, titanium bromide, cerium sulphide, etc.

If, for example, the inner element 3 of a combustion chamber consists of silicon carbide, it is resistant to temperature of 1450 C. in air. In a damp atmosphere intensive oxidation results in its rapid destruction. The protective skin of silicon oxide forming on the surface is permeable at elevated temperatures and breaks down so that the protective effect of the silicon oxide is lost. To obtain improved protective efiect and resistance to elevated temperatures, a pre-melted and subsequently pulverized mixture of K 0, A1 and Si0 for example, is injected from the mixing tank 15 through the feed pipe 23 and the header 13 and the injection tubes 11 into the combustion chamber, so that the inner wall element is covered with a stable dense glass-like coating. Adjustment of the powder mixture as regards melting point and composition gives protection to above 2,000 C. in the case of silicon carbide. In this way, the Wear of the innermost layer of the inner wall element 3 can be compensated from time to time by the application of such a coating so that the life of the wall 1 and hence of the entire combustion chamber is quite considerably increased.

It is possible to replace pure ceramics by cermets (i.e., ceramics-metals) for example oxide cermets, l e-A1 0 Al O -Cr, Cr-W-Al O Cr-Mo-TiAl O or other oxide cermets in the form of A1 0 and BeO with Co, Ni, Fe, W-Cr; BeO-Nb, ThO -Mo and ZrO -Ni, CaO-Co.

The metallic materials which are suitable for injection into the combustion chamber and for the formation of a coating on the inner wall element 3 are, for example high-' chromium-content alloys and the metals, chromium, titanium, etc. Chromium has a melting point of 1900 C. and compared with compact-ceramic materials has a much better thermal conductivity. The chromium oxide (Cr O forming on the chromium has the following very favorable properties as regards wall coating:

The transition from the metallic chromium to Cr O is rarely dense and has very good adhesion at the metaloxide transition layer.

The melting point of C-r O at 1900-2200 C. is relatively high.

Cr O is per se very resistant to corrosion.

Cr O together with bivalent metal oxides, for example MgO, ZnO, FeO, NiO, BeO and the like, forms spinels of the MgO.Cr O type.

All the high-temperature oxides very readily form compounds with Cr O for example Al O .Cr O

Thus, there may be used for the injection powders various ceramic powders, powder mixtures, pre-melted and pulverized melts of all kinds and cermet powders such as carbides, borides, nitrides, sulphides and metal powders. They can be injected singly, in mixture, optionally in succession, etc.

It is also possible, for example, to inject a high-melting oxide, such as MgO, ZrO etc., together with a low-melting material, such as SiO LiF, BaO, ZnO or mullite, or else the high-melting oxide, carbide, boride, etc., can first be coated in a low-melting flux and then be conveyed to the interior of the combustion chamber. Caking or sintering on the inner wall of the combustion chamber can be very considerably facilitated in this way in individual cases. The flux will evaporate during operation or form a new ceramic compound which can be controlled within predetermined limits.

In this way, for example, a jacket or coating of LiF-l- 0.4 mol percent ZrO can be provided for the MgO particles with a melting point of 2800" C. before injection by the sintering and melting process. The core of the particle still has a melting point of 2800 C. but the particle jacket has a melting point of only about 1600-1800 C. This facilitates sintering of the powder on the hot combustion chamber wall of MgO for example. At the high operating temperature the MgO particles melt very rapidly on the inner surface of the hot wall of the inner element 3. The flux in the particle jacket very rapidly evaporates after the sintering on the wall and the total melting point of the surface which is sintered at operating temperature thus immediately rises again to 2800" C.

FIG. 3 shows a combustion chamber wall 44 for the construction of a cylindrical combustion chamber. It will comprises an inner wall element 46 and an outer wall element 48. The inner wall element 46 is a sintered element made from cylinders 50 while the outer wall element 48 is made up of shaped bricks 52. Cooling is effected by the injection of cooling medium through the injection tubes 11, appropriate powders or powder mixtures being added to the cooling medium as explained above, in order to coat the inner wall element 46.

The air emerges into the combustion chamber through the interstices between the cylinders 50. The size of the air passages and the number thereof can be controlled by the choice of diameter for the cylinders 50.

Combustion chamber element 60 (see FIG. 4) comprising an inner wall part 62 and an outer wall part 64 can also be so sintered that the inner wall part 62 remains porous and hence air-permeable while the outer wall part 64 is dense. An injection tube 66 is introduced through the dense outside wall part 64 to the porous inner part 62 for the supply of cooling air and the powder to build up the inner wall of the combustion chamber. The fine structure of the inner wall part 62 insures adequate and good distribution of the air and entrained powder so that the inner wall is very uniformly swept by air and powder on the inside of the combustion chamber.

FIG. 5 shows a construction in which a combustion chamber element 70 has an inner wall element 72 to whose outside an air chamber 74 is connected, such chamber being enclosed by a shaft wall 76. Cooling air tubes 78 feed the cooling air to the chamber 74 while a ceramic dust feed pipe 80 supplies the required internal wall coating substance for the element 72, together with cooling air as a vehicle. The mixture of dust and air is blown to the interior of the combustion chamber through the fine pores of the porous element 72 and serves to cool the combustion chamber wall and restore the material thereof as already described.

The inner wall element may in principle be constructed from sintered fragments, cylinders, balls or other regular or irregular shaped bodies. The important feature is that the number of pores and width should be such that uniform cooling of the surface swept by the hot medium is insured with a minimum of air and a uniform coating is obtained by the appplica-tion or sintering of injected powder on the wall.

It has been found advantageous to arrange the cooling tubes slidably in the heat-resistant element as illustrated in FIG. 2.

I claim:

1. A method of protecting the surface of a gaspermeable heat-resistant element that is to be exposed to high temperature which comprises flowing a suspension of a finely divided solid in a cooling gas through said element to said surface.

2. A method as defined in claim 1 in which the finely divided solid is a ceramic material.

3. A method as defined in claim 1 in which the finely divided solid consists essentially of at least one metallic oxide.

4. A method as defined in claim 3 in which the finely divided solid is a mixture of K 0, A1 0 an SiO 5. A method as defined in claim l in which the finely divided material is at least one metal.

6. A method as defined in claim 5 in which the finely divided material is a high chromium alloy.

6 7. A method as defined in claim 5 in which the finely References Cited by the Examiner divided material is a cermet.

UNITED STATES PATENTS 8.A thd dfind' l' 5' h'hth fil me O as e e m c mm m W a ne y 2,941,759 6/1960 Rice et a1. 165134 divided material is titanium.

9. A method as defined in claim 1 in which the finely 5 I divided solid is a high melting material coated with a MEYER PERLIN P'lmary Examme" low melting material. CHARLES SUSKO, Examiner. 

1. A METHOD OF PROTECTING THE SURFACE OF A GASPERMEABLE HEAT-RESISTANT ELEMENT THAT IS TO BE EXPOSED TO HIGH TEMPERATURE WHICH COMPRISES FLOWING A SUSPENSION OF A FINELY DIVIDED SOLID IN A COOLING GAS THROUGH SAID ELEMENT TO SAID SURFACE. 